New research suggests that teenagers who exercise are less likely to develop symptoms of asthma - a finding that appears to be largely explained by their lower rates of smoking.

In a study of nearly 3,000 German teenagers, researchers found that those who exercised at least once a week were less likely to develop wheezing problems. The relationship disappeared, however, when the researchers factored in smoking, a known risk factor for asthma.

Teens who exercised were less likely than their sedentary peers to smoke. Similarly, those who said they used their computers more than one hour a day had a lower likelihood of wheezing - again, explained mainly by lower smoking rates.

“For children and teenagers, smoking, whether active or passive, is a well-recognized risk factor for asthma,” lead study author Dr. Christian Vogelberg, of University Children’s Hospital in Dresden, said in a statement.

“This link with smoking is emphasized by the fact that the connection between physical activity and asthma disappears when active smoking is factored in,” he explained.

In fact, teenagers who smoked were 2.5 times more likely to develop wheezing than their non-smoking peers, Vogelberg and his colleagues report in the European Respiratory Journal.

The researchers based their findings on data from 2,910 students who were surveyed twice - first when they were 9 to 11 years old, then again when they were between 16 and 18 years old.

None reported any problems with wheezing during the first survey, but by the second survey 11 percent had developed wheezing symptoms. Some of these teens had also been diagnosed with asthma.

At first look, both regular exercise and regular computer use were related to a lower risk of wheezing. But neither link held up when the researchers factored in smoking.

It’s not clear, however, whether this means that exercise actually makes teenagers less likely to take up smoking - and, therefore, indirectly lowers the risk of asthma, according to Vogelberg and his colleagues.

“Our study’s methodology did not allow us to determine whether a teenager was a non-smoker because he liked sport or whether he was inactive because he smoked or was subjected to passive smoking,” Vogelberg explained.

Future studies, the researchers conclude, will need to separate the independent effects of exercise and smoking on asthma risk.

SOURCE: European Respiratory Journal, October 2007.